Messages in this thread | | | Date | Tue, 15 Nov 2011 23:08:21 -0800 | From | Paul Turner <> | Subject | Re: [PATCH] sched: avoid unnecessary overflow in sched_clock |
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On 11/15/2011 10:41 PM, Mike Galbraith wrote: > On Tue, 2011-11-15 at 15:02 -0800, john stultz wrote: >> On Tue, 2011-11-15 at 14:12 -0800, Salman Qazi wrote: >>> (Added the missing signed-off-by line) >>> >>> In hundreds of days, the __cycles_2_ns calculation in sched_clock >>> has an overflow. cyc * per_cpu(cyc2ns, cpu) exceeds 64 bits, causing >>> the final value to become zero. We can solve this without losing >>> any precision. >>> >>> We can decompose TSC into quotient and remainder of division by the >>> scale factor, and then use this to convert TSC into nanoseconds. >>> >>> Signed-off-by: Salman Qazi<sqazi@google.com> >> >> Acked-by: John Stultz<johnstul@us.ibm.com> > > This wants a stable tag, no? > > -Mike >
Probably a good idea -- This especially sucks rocks in the sched_clock_stable==1 case; resulting in it coming straight back out of sched_clock_cpu() and trashing rq->clock.
- Paul
>>> --- >>> arch/x86/include/asm/timer.h | 23 ++++++++++++++++++++++- >>> 1 files changed, 22 insertions(+), 1 deletions(-) >>> >>> diff --git a/arch/x86/include/asm/timer.h b/arch/x86/include/asm/timer.h >>> index fa7b917..431793e 100644 >>> --- a/arch/x86/include/asm/timer.h >>> +++ b/arch/x86/include/asm/timer.h >>> @@ -32,6 +32,22 @@ extern int no_timer_check; >>> * (mathieu.desnoyers@polymtl.ca) >>> * >>> * -johnstul@us.ibm.com "math is hard, lets go shopping!" >>> + * >>> + * In: >>> + * >>> + * ns = cycles * cyc2ns_scale / SC >>> + * >>> + * Although we may still have enough bits to store the value of ns, >>> + * in some cases, we may not have enough bits to store cycles * cyc2ns_scale, >>> + * leading to an incorrect result. >>> + * >>> + * To avoid this, we can decompose 'cycles' into quotient and remainder >>> + * of division by SC. Then, >>> + * >>> + * ns = (quot * SC + rem) * cyc2ns_scale / SC >>> + * = quot * cyc2ns_scale + (rem * cyc2ns_scale) / SC >>> + * >>> + * - sqazi@google.com >>> */ >>> >>> DECLARE_PER_CPU(unsigned long, cyc2ns); >>> @@ -41,9 +57,14 @@ DECLARE_PER_CPU(unsigned long long, cyc2ns_offset); >>> >>> static inline unsigned long long __cycles_2_ns(unsigned long long cyc) >>> { >>> + unsigned long long quot; >>> + unsigned long long rem; >>> int cpu = smp_processor_id(); >>> unsigned long long ns = per_cpu(cyc2ns_offset, cpu); >>> - ns += cyc * per_cpu(cyc2ns, cpu)>> CYC2NS_SCALE_FACTOR; >>> + quot = (cyc>> CYC2NS_SCALE_FACTOR); >>> + rem = cyc& ((1ULL<< CYC2NS_SCALE_FACTOR) - 1); >>> + ns += quot * per_cpu(cyc2ns, cpu) + >>> + ((rem * per_cpu(cyc2ns, cpu))>> CYC2NS_SCALE_FACTOR); >>> return ns; >>> } >>> >>> >> >> >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in >> the body of a message to majordomo@vger.kernel.org >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> Please read the FAQ at http://www.tux.org/lkml/ > >
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